Ultra high frequency resonator



Patented Oct. 5, 1943 ULTRA HIGH FREQUENCY RESONATOR John A. Hutcheson, Catonsville, Md., assignor to Westinghouse Electric & Manufacturing Company, East Pittsburgh, Pa, a corporation of Pennsylvania Application January 14, 1942, Serial No. 426,689

Claims.

My invention relates to electrical discharge tubes and circuits therefor, and in particular to tubes and circuits of this type which are designed to operate at extremely high frequencies.

One object of my invention is to provide a new type of tuned electrical circuit and, in particular, one in which the tuning may be varied and which has a very low value of decrement.

Another object of my invention is to provide an arrangement for efficiently cooling the anode of an electrical discharge device.

Still another object of my invention is to provide a structure for supporting an electrical discharge device and for efliciently connecting it to an oscillation circuit of the concentric line type.

Another object of my invention is to provide a structure which performs the combined functions of eficiently cooling the anode of an electrical discharge device while coupling it for.efilcient operation in conjunction with an oscillation circuit of the concentric line type.

Still another object of my invention is to provide an electrical discharge device adapted to act as an oscillator, amplifier or detector of radio currents of extremely high frequencies with a composite structure which performs the functionsoi supporting the discharge tube, efficient- 1y cooling its ari'bde and providing a tunable oscillation circuit of the concentric line type which is so coupled to the discharge tube as to permit efiicient operation in conjunction therewith.

Other objects of my invention will become apparent on reading the following specification taken in conjunction with the drawing in which the single figure illustrates an electrical discharge tube provided with a composite cooling device and oscillation circuit embodying the principles of my invention.

Electrical discharge tubes which are designed to amplify, oscillate or detect alternating currents of extremely high frequencies, such, for example, as those radiating waves having a length of the order of one meter usually have relatively small physical dimensions; and, where they are to transmit even moderate amounts of power, a problem arises in attaining eilicient dissipation of the heat set free at their anodes. One typical tube of the type just discussed has an anode comprising a copper shell forming a portion of the tube wall, the control electrode and cathode being positioned in the interior of this shell and sealed thereto by a glass wall portion positioned at one end of the copper shell. The anode shell constitutes a major portion of the side walls and also forms one end of the tube, and the cathode and grid leads project from this glass wall portion at the other end of the tube.

In accordance with the prior art, radial vanes of considerable dimensions were attached to the copper anode shell for the purpose oi dissipating the heat set free at the anode during operation of the tube. Eiiicient operation of such a tube for generating or amplifying ultra-short waves likewise requires that the length of the electrical circuits traversed by the ultra-short current shall be reduced to relatively small values, and the stray capacitance to ground and between portions of the circuit should be reduced to the minimum possible value. Because of the difficulties arising from stray capacitance, it has been found that that oscillation circuits comprising variable plate-condensers and inductances which are conventional in ordinary radio circuits, create difliculties and that much more satisfactory operation for ultra-short wave work results if oscillation circuits of what may be called the concentric transmission-line type are employed. Such oscillation circuits may comprise a tubular conductor having a cylindrical conductor of smaller diameter positioned along its axis, the two conductors being connected with each other by a plate at one end.

A Patent No. 2,228,126 to Sheldon L Rambo, filed August 12, 1938, for High frequency coupling condensers and assigned to the Westinghouse Electric 8: Manufacturing Company of East Pittsburgh, Pennsylvania, illustrates the use of an oscillation circuit of the concentric line type in combination with an electrical discharge tube of the type which has just been described. To utilize the concentric line as an oscillation circuit it is electrically coupled to the anode of the electrical discharge device, and in order to minimize stray capacitance and other sources of inefiicient operation, it is desirable to make the connection between the concentric line and the anode of the discharge device as short and compact as is possible. In the Rambo arrangement the expedient has been resorted to of surrounding the radiating fins which cool the anode of the discharge tube device by a dielectric sheet in contact at its outer periphery with a metal sheet which is directly connected to the centr conductor oi the concentric line. In this arrangement the dielectric sheet in effect constitutes the structure the dimensions of this condenser are considerable, resulting in considerable stray capacitance, a large energy storage in the capacitor and a considerable power loss in the capacitor dielectric and elsewhere. Furthermore, the capacitance inherent in these large dimensions constitutes a fixed capacitance which amounts practically to a limitation on the magnitude of the highest frequency to which it is possible to tune the circuit.

In accordance with my present invention I avoid the use of radial cooling fins on the anode of the discharge tube but instead employ the structure of the concentric line, in effect, as a cooling device to dissipate the heat set free at the tube anode.

In this way I am enabled to further decrease the linear dimensions of the entire ultra-short wave system as well as to avoid the dielectric and other losses inherent in the capacitor coupling employed in the Rambo system.

In accordance with another feature of my invention, I provide a structure for an oscillation circuit of the concentric line type which is adapted to form a resonant circuit having a very low decrement and an extremely low power factor, and which may be tuned to resonate at various frequencies. This resonant circuit comprises, as is shown in the drawing, a central core of conducting material concentrically surrounded by an outside shell or tube of conducting material, the core and shell being connected together at the end remote from the oscillation-generating tube by a conductive disk. A plunger or disk of conducting material slides in conductive engagement along the central core, but its periphery is separated from the outside cylinder by an air gap of dimensions which will be discussed below. The concentric line is tuned to resonate by displacing the plunger or disk axially along the central core.

It can be shown that such a concentric line resonator as has just been described can be made to oscillate in at least two different modes of vibration, both of which probably exist simultane ously in some degree, but the relative amplitude of which is determined by certain dimensions, notably by the width of the gap between the sliding plunger and the surrounding cylinder.

If the gap between the plunger and the surrounding cylinder is made extremely small compared with the radial distance separating the outer surface of the core from the inner surface of the cylinder, and small relative to the axial length of the cylindrical surface of the plunger itself, the unit will vibrate predominantly as a quarter-wave resonator in the portion of the unit intervening between the plunger and the open end (i. e., that adjacent the oscillation generator) of the surrounding tube. That is to say, there will be a vibration having a wave length approximately four times the distance from the plunger to the open end of the surrounding tube which will be of predominant amplitude in the system. In such an arrangement, the capacitance between the plunger and the external tube acts very much like a short-circuit for the resonant frequency, and the frequency of this predominant vibration is little affected by the portion of the tube between the plunger and the closed end of the unit. In fact, this closed end of the unit might probably be entirely removed without aii'ecting the resonant vibration in the system. In such a system as that which has just been described, where the radial length of the gap between the plunger and the surrounding cylinder is made relatively small, the reactance at the maximum frequency at which the unit is expected to resonate would 5 be of the order of or less of the surge impedance of the concentric line. This surge impedance is, as is well known, equal to the square root of the inductance per unit axial length of the line divided by the capacity per unit axial 1 length existing between the central core and the surrounding cylinder.

If in contrast to the foregoing the gap between the plunger and the surrounding cylinder is made of substantial width, there will be found to exist in in the unit a predominating vibration of a different type than that described above; namely, there will exist a vibration in which the frequency is determined by the distance between the plunger and the closed end of the concentric line, i. e. that remote from the oscillation generating tube. In

such a case, the portion of the surrounding cylinder between the plunger and the open end of the external tube plays a minor part in the vibration. The frequency of this predominating vibration is no such simple function of the distance between the plunger and one end of the tube as is that given above for the arrangement in which the radial gap between the plunger and the external tube is small, and the precise expression relating this frequency to the diameter of the core and of the external cylinder and the axial displacement of the plunger need not be given to understand the present invention. It is sufficient to say that by displacing the plunger along the axis of the concentric line, a resonant vibration of low decrement and high power factor will be set up in the portion of the line between the plunger and the closed end, and that the frequency of this vibration may be varied by varying the axial displacement of the plunger.

While I do not wish to be bound by this or any other theoretical explanation of the mode of operation of the structure which I have invented, r I may state it is my opinion that vibrations of the two types so far described coexist with different degrees of amplitude in a structure having a gap of any radial width between the plunger and the surrounding cylinder, but that the relative amplitudes of the two modes of vibration depend upon the radial width of this gap; the first type described being more predominant over the second type described as the radial width of this gap is decreased, and vice versa, the second type i of vibration becoming more and more predominant relative to the'first type of vibration as the radial width of this gap is increased. While the transition from conditions in which the one type predominates to conditions in which the other type predominates is, I believe, a continuous and gradual one as the width of the gap is varied, I may state, as typical values, that the first-mentioned type of vibration predominates where the capacitive reactance between the periphery of the plunger and the surrounding cyl- 6 inder is 10% or less of the surge impedance of the concentric line structure, whereas the second mode of vibration is greatly predominant in amplitude where the capacitance between the periphery of the plunger and the surrounding cylinder is such that the reactance at the maximum resonant frequency is equal to or greater than 50% of the surge impedance. It will be evident that the maximum frequency of resonance for the first-mentioned mode of vibration exists when the plunger is positioned as closely as possible to the open end of the enclosing cylinder, and that the maximum frequency of vibration for the second-mentioned mode of vibration occurs when the plunger is displaced to the limit of its travel toward the closed end of the cylinder.

With the foregoing principles in mind, the drawing shows an electrical discharge tube I having an anode 2 comprising a copper shell forming the wall over the entire upper end of the tube. A control electrode 3 and an incandescent cathode 4 of conventional type are located in the interior of the anode portion 2 and are sealed vacuum-tight to the lower end thereof by means of glass in a manner too well known in the art to require description. Current to heat the cathode 4 is supplied from a transformer having a grounded casing and the cathode leads are grounded through two condensers 6, 1 near their point of entrance to the tube electrode 3 is connected to one terminal of a conventional input circuit, the other terminal of which may be grounded and this ground is connected to the grid through a condenser 8 and a resistor 9. The connections of the tube I so far described are conventional ones and may be replaced by any other suitable connections for impressing the voltage of an input circuit between the cathode 4 and the control electrode 3.

The tube I may conveniently be mechanically supported by the concentric line structure comprising a grounded metallic tube II which may be ailixed to a grounded supporting frame. The tube may have one end closed by a metal plate l2 which supports in the central axis of the tube H a cylindrical core I3. The end of the core l3, which is opposite to the plate I2, is machined out to receive with a perfect fit the anode 2 of the tube I. Since the core l3 tightly fits the tube anode 2, it constitutes an efilcient means for conducting away the heat dissipated within said anode during operation of the tube I. Heat flows from the anode I longitudinally of the core l3 to the opposite end thereof which is provided with a series of radiating vanes I4 to help in maintaining the temperature of the end plate l2 as near to that of the room as possible. Some of the heat flowing along the core l3 flows into the end plate I2 and from the latter into the tube ll so that these last-named elements likewise help in dissipating the heat generated within the anode of tube In order to tune the concentric line consisting of tube l I, anode plate l2 and core I3, a structure comprising a collar IS, a disk It, and an annular ring H is arranged to slide along the core l3. The outer diameter of the ring I1 is made slightly less than the internal diameter and the tube H, as a result of which the structure l5, l6, I1 is capacitively coupled to the tube II by the annular spacing last mentioned. The collar I5 forms a tight sliding fit with the core l3 and if necessary to maintain good electrical contact thereto, sliding brushes such as element Ill may be provided in a manner well known in the electrical art.

When the collar l5, l6, H is displaced along the core l3, the space between the disk I6 and the end plate I2 is traversed by an electrical field and the entire arrangement constitutes a tuned oscillating circuit having an extremely low decrement. The wave length of this electrical circuit is at a minimum when the disk I6 is adjacent the end plate l2 and is at a maximum when the disk I! is adjacent the open end of the tube H.

The control In order to tune the concentric line just described, provision is made for sliding the collar l5 along the core Ill. The adjusting arrangement comprises a rod 2| which may be of metal, or may be of some insulating material such as Micarta, which is preferably of a very small diameter, and which extends through a small hole bored in the central axis of core l3. Two deep and narrow slots connecting the central hole in member I! with its periphery extend for a considerable distance of the length of member l3, and permit a pin 22 which is set in the end of the rod 2| and extends through holes in the collar l5 to move the latter along the core l3 when a push or pull is exerted on the outer end of the rod 2|.

To energize the electrical circuits of the tube a source of voltage 23 is connected through a choke ,coil 2 with the lower end of the rod |3.

Since most of the heat generated in the anode 2 flows longitudinally along the core I3, it is highly desirable that the impedance to heat flow of the latter shall be minimized. For this reason the core I3 is preferably made of copper or some other good heat conductor and the size of its central hole and appurtenant slots is made as small as possible. The temperature difference between the lower and the upper end of the core l3 may be kept to any desired low value increasing the diameter of the core I3 to the necessary diameter.

To give a. specific illustration of one embodiment of my invention, I have iounr. that Where the tube l was of the type sold under the trade designation WX-3005 and operated to dissipate 150 watts of power at its anode in conjunction with a core l3 having a length of 12 inches and a diameter of 1.6 inches, the temperature diiierence between the upper and lower ends of the core l3 was less than C. with ordinary room temperatures surrounding the apparatus. This provides a safe operating temperature for the tube I.

This application is a continuation-in-part of my application Serial No. 357,355, filed September 19, 1940, now Patent No. 2,285,662, dated June 9, 1942, and assigned to the Westinghouse Electric. & Manufacturing Company of East Pittsburgh, Pennsylvania.

In accordance with the patent statutes I have described a specific embodiment of my invention, but the principles thereof will be applicable in many other ways which will be evident to those skilled in the art.

I claim as my invention:

1. In combination with a source of high frequency alternating currents, an electrical resonator comprising a central core of conducting material having concentric therewith a cylinder of conducting material, a conducting plate interconnecting said core and said cylinder at one end, a movable plunger of conducting material in contact with said central core, and having an air gap between its periphery and the internal face of said cylinder oi. such length that frequency of the predominant electrical resonant vibration of the system is determined by the electricrl constants of the portion of said structure between said plunger and said plate.

2. In combination with a source of high frequency alternating currents, an electrical resonator comprising a central core of conducting material having concentric therewith a cylinder of conducting material, a conducting plate interconnecting said core and said cylinder at one end, a movable plunger of conducting material in contact with said central core, and having a gap between the periphery or said plunger and the internal face of said cylinder of such width and area that the capacitance constituted by said gap has a reactance when said plunger is at its limit of travel toward said plate which is of the order of 50% of the surge impedance of said core and surrounding cylinder.

3. In combination with a source of high trequcncy alternating currents, an electrical resonator comprising a central core of conducting material having concentric therewith a cylinder of conducting material, a conducting plate interconnecting said core and said cylinder at one end, a movable plunger of conducting material in contact with said central core, and having a gap between the periphery of said plunger and the internal face of said cylinder of such width and area that the capacitance constituted by said gap has a reactance when said plunger is at its limit of travel toward said plate which is of the order of 50% of the surge impedance of said core and surrounding cylinder, and means for varying the axial distance between said plunger and said plate.

4. In combination with a source of high frequency alternating currents, a resonant circuit unit comprising a central core of conducting material surrounded by a concentric cylinder having conducting walls, a conducting plate interconnecting one end of said cylinder with said central core, a plunger comprising a cylindrical conducting suriace concentric with said cylinder and supported for sliding conductive connection with said core, the radial gap between said plunger and said cylinder being large enough so that the predominant electrical vibration at resonance of said unit has a frequency determined by the axial distance between said plunger and said plate.

5. In combination with a source of high frequency alternating currents, a resonant circuit unit comprising a central core of conducting material surrounded by a concentric cylinder having conducting walls, a conducting plate interconnecting one end of said cylinder with said central core, a plunger comprising a cylindrical conducting surface concentric with said cylinder and supported for sliding conductive connection with said core, the radial gap between said plunger and said cylinder being large enough so that the predominant electrical vibration at resonance of said unit has a frequency determined by the axial distance etween said plunger and said plate, and means for slidably displacing said plunger along said core.

JOHN A. HUTCHESON. 

